The subject matter disclosed herein relates generally to reducing radiated emissions in switching power converters and, more specifically, to a motor drive including a filter which reduces emissions radiated from the motor leads.
As is known to those skilled in the art, motor drives permit variable speed control of motors that would otherwise run at a single speed if connected directly to a power source. Motor drives include many configurations, but a common configuration includes a rectifier section, which converts an alternating current (AC) power source into a direct current (DC) voltage. The DC voltage is transferred to a DC bus having a capacitance connected across the bus to reduce the ripple voltage resulting from rectifying the AC power into a DC voltage. The DC voltage is subsequently provided as an input to an inverter section, which converts the DC voltage into an AC voltage of varying frequency and magnitude according to the operating requirements of the motor.
In order to generate the AC voltage of varying frequency and magnitude, the motor drive modulates, or switches, the DC voltage on and off at a periodic interval to output a desired average value of voltage over the interval. A modulation period is selected over which the desired average voltage is output. The modulation period is the inverse value of the modulation frequency, which is dependent on factors such as the power rating, topology, or the modulation technique of the motor drive and may be, for example, between 1-15 kHz. The motor drive connects the DC bus to the output for a percentage of the modulation period. Thus, the output voltage is either equal to zero volts or to the voltage level on the DC bus; however, the resulting average value of the output voltage over the modulation period is equal to the voltage level on the DC bus multiplied by the percentage of the modulation period for which the output is connected to the DC bus. Because the modulation frequency is much greater than the desired AC output frequency of the motor (e.g., between 0 and 60 Hertz), the resulting average output voltage resembles an AC output voltage at the desired operating frequency of the motor.
The rapid switching of the output to the DC bus, however, can generate electrical currents resulting in radiated emissions from a cable connecting the output of the motor drive to the motor being controlled. In order to generate the desired average value of output voltage as previously discussed, it is desirable to rapidly turn on and off the solid state switching device. The solid state switching devices are, therefore, turned on and off within tens or hundreds of nanoseconds. In addition, some overshoot and subsequent oscillation as the output voltage settles may generate electrical signals in the tens or hundreds of megahertz, which are present at the output of the motor drive.
It is desirable to limit the magnitude of radiated emissions such that the emissions do not interfere with other electronic equipment. Presently, it is known to reduce the magnitude of the radiated emissions by including either a ferrite core or a common mode inductor on the output of the motor drive. However, both the ferrite core and common mode inductor are connected to the output of the motor drive. The wire gauge of the electrical conductors needed to carry the rated output current from the motor drive necessarily increase as well. However, the magnitude of the radiated emissions does not necessarily increase in a corresponding manner. For example, the magnitude of the radiated emissions may be in the microamps, but the output current of the motor drive may be in the tens or hundreds of amps. Because the ferrite core and the common mode inductor are in series with the output of the motor drive, they must be sized to handle the conductors on the output of the motor drive and similarly increase in size and expense as well.